There is a continuing need for development of new anticancer drugs, drug combinations, and chemotherapy strategies. To spur development of new cancer drugs, a screening and discovery program for cancer chemotherapeutics was established at the National Cancer Institute (NCI) in 1960. Screening of plant extracts began with a survey of the flora of the U.S. conducted in collaboration with the U.S. Department of Agriculture (S. A. Schepartz, Cancer Treat. Repts, 60, 975 (1976) and J. A. Hartwell, Cancer Treat. Repts, 60, 1031 (1976)). Taxus brevifolia Nutt. (Family Taxaceae), the Pacific yew or Western yew, was collected in 1962 as part of this program from Washington State. The Pacific yew is a smallish, slow growing tree native to the Pacific Northwest with a North-South range from Southeastern Alaska to Northern California, and extending eastward to mountainous areas of Idaho and Montana. It is often found as an understory tree in populations of Douglas fir. The Taxaceae is a small, somewhat isolated, botanical family with 5 genera of which Taxus is the most prominent with eleven species worldwide.
Taxol is part of the family of chemical compounds known as Taxanes. See M. Suffness, "Chapter 34. Taxol: From Discovery to Therapeutic Use," Annual Reports and Med. Chem. (in print) and U.S. Pat. No. 5,248,796 to Chen, et al., which are hereby incorporated by reference.
Taxol was found to be active clinically against advanced ovarian and breast cancer, (Towinsky, E. K., Cazenave, L. A., and Donehower, R. C., "Taxol--a Novel Investigational Antimicrotubule Agent," J. Nat. Canc. Inst. 2:1247-59 (1990)). In phase II trials, the response rate was 30% in heavily-pretreated patients with advanced and refractory ovarian cancer, (McGuire, W. P., Rowinsky, E. K., Rosenshein, N. B., Grumbine, F. C., Ettinger, D. S., Armstrong, D. K. and Donehower, R. C., "Taxol: a Unique Antineoplastic Agent with Significant Activity in Advanced Ovarian Epithelial Neoplasms," Ann. Intern. Med. 111:273-279, (1989)). The overall response rate was 56% in phase II trials in pretreated patients with metastatic breast cancer, (Holmes, F. A., Walters, R. S., Theriault, R. S., Forman, A. D., Newton, L. K., Raber, M. N., Buzdar, A. U., Frye, D. K., and Hortobagye, G. N., "Phase II of Taxol, an Active Drug in the Treatment of Metastatic Breast Cancer," J. Natl. Cancer Inst., 83:1797-1805, (1991)). Recently, the U.S. Food and Drug Administration approved taxol for use against ovarian cancer.
Because of its poor solubility in water and in most pharmaceutically-acceptable solvents, the formulation selected for clinical administration consists of taxol solubilized in Cremophor EL.RTM. (polyethoxylated castor oil) containing 50% absolute ethanol ("Diluent 12"). The amount of Cremophor necessary to deliver the required doses of taxol is significantly higher than that administered with any other marketed drug. This vehicle has been shown to cause serious or fatal hypersensitivity episodes in laboratory animals (Lorenz, W., Riemann, H. J., and Schmal, A., "Histamine Release in Dogs by Cremophor EL and its Derivatives: Oxyethylated Oleic Acid is the Most Effective Constituent," Agents Actions 7:63-7, (1977)) and humans (Weiss, R. B., Donehower, R. C., Wiernik, P. H., Ohnuma, T., Gralla, R. J., Trump, D. L., Baker, J. R., VanEcho, D. A., VonHoff, D. D., and Leyland-Jones, B., "Hypersensitivity Reactions from Taxol," J. Clin. Oncol. 8:1263-8 (1990)). Since hypersensitivity reactions appear to occur more frequently with shorter infusion schedules, most phase II and III trials in the United States have used 24-hour schedules (Rowinsky, E. K., Onetto, N., Canetta, R. M., and Arbuck, S. G., "Taxol: The First of the Taxanes, an Important New Class of Antitumor Agents," Seminar Oncol., 19:646-62 (1992)). Moreover, premedication with corticosteroids (dexamethasone) and antihistamines (both H1 and H2 receptor antagonists) is being used to reduce the intensity and incidence of reactions associated with taxol-Cremophor administration. Although the premedication regimen has reduced the incidence of serious hypersensitivity reactions to less than 5%, milder reactions still occur in approximately 30% of patients (Weiss, R. B., Donehower, R. C., Wiernik, P. H., Ohnuma, T., Gralla, R. J., Trump, D. L., Baker, J. R., VanEcho, D. A., VonHoff, D. D., and Leyland-Jones, B., "Hypersensitivity Reactions from Taxol," J. Clin. Oncol. 8:1263-68 (1990) and Runowicz, C. D., Wiernik, P. H., Einzig, A. I., Goldberg, G. L., and Horwitz, S. B., "Taxol in Ovarian Cancer," Cancer 71:1591-96 (1993)). Clinically, pharmacological intervention is less desirable than a safer, better-tolerated formulation; when several drugs are administered simultaneously, drug interactions that may affect taxol's efficacy or toxicity are more likely.
In view of the above-noted problems with taxol, researchers have sought to reformulate it in a better tolerated vehicle. Amongst these efforts is the use of liposomes to deliver taxol. In J. Riondel, et al., "Effects of Free and Liposome--With Encapsulated Taxol on Two Brain Tumors Xenografted into Nude Mice," In Vivo, 6:23-28 (1992), taxol was entrapped in soybean phosphatidyl choline and administered by intraperitoneal injection into mice with tumors. In M. H. Bartoli, et al., "In Vitro and In Vivo Antitumoral Activity and Free, and Encapsulated Taxol," J. Microencapsulation, 7(2):191-97 (1990), taxol encapsulated by liposomes was administered to cells and to animals by intraperitoneal administration to study antitumoral activity. Liposomes were formed from phosphatidyl choline. U.S. Pat. No. 4,534,899 to Sears includes an example where taxol is entrapped with soy phosphatidyl ethanolamine succinyl polyethylene glycol monomethylether, a synthetic phospholipid analogue.
Such prior work with liposomes has not yielded a system which delivers taxol safely and effectively and which is suitable for rapid administration directly into the bloodstream (i.e. intravenously). Applicants have discovered that the liposomes in such systems tend to form aggregates which cannot satisfactorily deliver taxol. Electrostatically neutral liposomes have a tendency to aggregate. Taxol, a hydrophobic, membrane-active chemical, promotes this aggregation. Large aggregated masses of particles are unsuitable for intravenous administration. Another problem encountered when liposomes (and most excipients) are utilized to deliver taxol is that the formulation becomes unstable and crystals of taxol form that withdraw taxol out of the solution. The presence of such crystals makes the system unsatisfactory and, in fact, lethal for intravenous administration, because such crystals cannot pass through capillaries. The cause of death resulting from the administration of such masses is likely renal and pulmonary failure, owing to the blockage of blood supply to these vital organs. There thus remains a great need to develop better systems for taxol delivery.